1. Field of the Invention
The present invention relates to controlled self-ignition four-stroke internal-combustion engines.
2. Description of the Prior Art
Controlled self-ignition is a well-known phenomenon in two-stroke engines. This combustion type has advantages as regards emissions: low hydrocarbon and nitrogen oxides emissions are notably obtained. Furthermore, a remarkable cycle regularity is achieved during self-ignition combustion.
Self-ignition is a phenomenon that allows to initiatant of combustion by means of residual burned gases which remain in the combustion chamber after combustion.
Self-ignition is achieved by controlling the amount of residual gases and the mixing thereof with the fresh gases (not burned yet). The residual gases (hot burned gases) initiate the combustion of the fresh gases resultant from a combination of temperature and of presence of active species (radicals).
In two-stroke engines, the presence of residual gases is &lt;&lt;inherent&gt;&gt; in the combustion. In fact, when the load of the engine decreases, the amount of fresh gases decreases, which leads to an increase in the amount of residual gases (burned gases from the previous cycle or cycles which have not flowed out of the cylinder). The two-stroke engine thus works with an internal recirculation (or internal EGR) of the burned gases at partial load. However, the presence of this internal EGR is not sufficient to obtain the desired self-ignition running. Research work also shows that mixing between this internal EGR and the fresh gases has to be controlled and limited.
The controlled self-ignition technology applied to four-stroke engines is particularly interesting because it allows the engine to run with an extremely diluted mixture, with very low fue/air ratios and ultra-low NOx emissions.
However, this technology comes up against a significant technological difficulty insofar as, in order to obtain controlled self-ignition in a four-stroke engine, it is necessary to either significantly increase the compression ratio of the engine (with knocking problems at high load), or to considerably heat up the fresh gases admitted (several hundred degrees C), or to combine these two phenomena.
Solutions allowing the decreasing of pressure and temperature level requirements for four-stroke engines can be partly provided by suitable additives in the fuel. French patent application FR-2,738,594 illustrates a solution of this type.
It is well-known for four-stroke engines, for example from international patent application PCT WO-93/16,276, to combine a variable distribution adjustment with a non-return system at the intake in order to reduce pumping losses at partial load. This solution then allows operation with the intake throttle as wide open as possible.
French patent application EN.97/02,822 filed by the assignee describes another way of controlling self-ignition in a four-stroke engine. More precisely, this document recommends, at partial load, to minimize mixing of the fresh gases and of the burned gases confined in the combustion chamber by delaying closing of the exhaust as much as possible. This is an &lt;&lt;internal&gt;&gt; recycle that allows stratification of the gases in the combustion chamber.
French patent application EN.97/11,279 filed by the assignee also minimized, at partial load, mixing of the fresh gases and of the burned gases contained in the combustion chamber, in order to control and to favor self-ignition combustion. However, this prior art proposes transferring the recycled burned gases via a specific line opening into an air supply line just upstream from the combustion chamber. The fresh air-fuel feed is introduced separately and late via a second line. A successive introduction of the feeds is thus provided. However, this solution creates a substantial dilution of the burned gases by air prior to entering the combustion chamber.